This invention relates to an apparatus for removing nitrogen oxides from a combustion flue gas discharged from boilers, industrial furnaces, gas turbines, and a combustion facility for treating wastes. Nitrogen oxides will be hereinafter referred to merely as NOx.
Recently in Japan, the fuel species for combustion is changing from heavy oil to coal due to tight supply of heavy oil to reduce the petroleum dependency, and coal-fired boilers of large capacity for power plants are now under construction for utility companies. However, coal as fuel has poor combustibility as compared with petroleum fuel, and NOx and uncombusted matters are liable to be emitted into the flue gas from coal-fired boilers. To reduce emission of NOx, slow coal combustion has been carried out by dividing the combustion flame into sections, or recycling the flue gas, conducting the combustion at two stages or removing Nox within the furnace before emission to the outside.
In the coal-fired power plants, the boilers are operated not always under a full load, but under a variable load on the order of 75%, 50% or 25% of full load, or the boiler operation is discontinued, for example, according to the so called Daily Start-Stop schedule (which will be hereinafter referred to merely "DSS schedule") or the so called Weekly Start-Stop schedule (which will be hereinafter referred to merely as "WSS schedule"). That is, the coal-fired power plants operatable under such intermediate load have been in keen demand.
On the other hand, a combination of a gas turbine of good startup characteristics with a waste heat recovery boiler, i.e. the so called combined plant, is now going to be contructed to meet the power generation under the intermediate load besides the coal-fired boilers, and is to be operated only in the day time from Monday through to Friday according to the DSS or WSS schedule to meet the large power demand, and the operation is stopped at night or on Saturday or Sunday or holidays.
However, according to more stringent restriction of the NOx concentration of flue gas, power plants operatable under the intermediate load, which are provided not only with the conventional combustion improvement, but also with an apparatus for removing NOx by catalytic reduction by dry process, i.e. an apparatus for removing NOx with NH.sub.3 as a reducing agent in the presence of a catalyst, have been now constructed in increasing numbers.
Particularly in the coal-fired boilers, the amount of NOx increases owing to the poor combustibility of the coal fuel itself, and also in the gas turbine plants, a large amount of NOx is contained in the flue gas, as in the coal-fired boilers, owing to a large amount of oxygen used and combustion at a higher temperature. That is, an apparatus for removing NOx, for example, as shown in FIG. 1, is usually installed in the power plant.
In FIG. 1, a typical flue duct for a boiler based on the equilibrium draft system, provided with an apparatus for removing NOx, is schematically shown.
Air for combustion in an air duct 1 is pressurized by a forcing draft fan (FDF) 2, heated by an air preheater 3 through heat exchange with the flue gas passing through a flue gas duct 4, and then supplied to a boiler 6 from a wind box (W/B) 5.
On the other hand, the combustion flue gas from the boiler 6 is passed through the flue gas duct 4, subjected to NOx removal by NH.sub.3 injected from a NH.sub.3 injection tube 7 while the removal of NOx is accelerated by a catalyst 9 in an NOx removal reactor 8, provided at the downstream side of the NH.sub.3 injection tube 7, and passed through the air preheater (A/H) 3 and an electrostatic precipitator (EP) 10 after the removal of NOx from the flue gas, pressurized by an inducing draft fan (IDF) 11 and vented to the atmosphere.
Reaction temperature range for the NOx removal reactor 8 in the apparatus somewhat depends on the species of catalyst 9, but the temperature range of highest NOx removal efficiency is a relatively high and very narrow, such as 300.degree. to 400.degree. C. In the boilers or gas turbines operatable under the intermediate load, which are always operated according to the DSS or WSS schedule, the flue gas temperature widely fluctuates, depending on changes in the load, and often fails to fall within the said applicable reaction temperature range of catalyst 9.
When the flue gas temperature is much higher than the reaction temperature of catalyst 9, the structure of catalyst 9 will change and the function of catalyst 9 is deteriorated, whereas, when the flue gas temperature is much lower, the catalyst 9 will react with sulfuric anhydride (SO.sub.3) existing in the flue gas, deteriorating the catalyst 9.
On the other hand, when tubes of boiler 6 are damaged, the operation of boiler 6 is stopped immediately, and the boiler 6 itself is forcedly cooled with the air for combustion from the air duct 4 by FDF 2, and steam or water leaked from the damaged tubes of boiler 6 is entrained with the combustion gas and discharged as a highly humid gas from the boiler 6. The discharged flue gas carries water mists below about 100.degree. C.
Deteriorating components such as alkali metals, etc. dissolved in the steam or water leaked from the tubes flow into the NOx removal reactor 8 through the flue gas duct 4 and deteriorate the catalyst 9.
The activity of the catalyst used in the reactor 8 is gradually lowered with time, as the operation of boiler 6 is continued, and thus it is necessary to provide a means for monitoring the activity of catalyst 9 to determine the timing of exchanging the catalyst 9 or regenerating the catalyst 9. When the performance of catalyst 9 is lowered by a sudden accident, etc. in the boiler 6, as described above, it is important to investigate causes to lower the catalyst performance and take an inmediate measure to cope with the causes. In any case, it is a key to investigate the performance of catalyst 9 itself. It is the ordinary expedient to sample the catalyst 9 periodically or when required to monitor or detect the deterioration in performance of catalyst 9. The catalyst 9 is usually packed in catalyst packages of integrated structure in the reactor 8 so that no clearances may be formed between the catalyst packages to prevent a gas leakage therebetween. Thus, it takes much labor and time to sample even a small amount of catalyst necessary for the monitoring and detection from the catalyst packages. As shown in FIG. 2, it is the ordinary expedient to provide a small number of catalyst test pieces 12 in the catalyst packages in the NOx removal reactor 8. To take the catalyst test pieces 12 out of the catalyst packages in the reactor 8, workers must enter the reactor 8. That is, the catalyst test pieces 12 can be taken out only after the flue gas has been completely shut off from the boiler 6 and also from the reactor 8 and the temperature within the reactor 8 has been cooled down to the ambient temperature. Thus, it takes much labor and time to take out catalyst test pieces 12 from the catalyst packages in the reactor 8. In the case of an emergency such as a boiler accident, etc., it is considerably delayed to take the necessary measure for preventing the deterioration of catalyst 9. Furthermore, discontinuation of operation of reactor 8 even for a few days for taking out catalyst test pieces 12 from the catalyst packages is not preferable from the viewpoint of environmental pollution, because NOx as bypassed is discharged into the atmosphere during the discontinuation of NOx removed reactor.
As shown in FIG. 2, the catalyst 9 is provided in a plurality of stages to facilitate exchanging of catalyst 9 in the case of catalyst deterioration. The flue gas passes through such a plurality of stages of the catalyst 9 at an equal flow rate, but since the catalyst-deteriorating components contained in the flue gas are adsorbed onto the catalyst 9 to some degree, the influence of the flue gas property on the catalyst activity differs between the catalyst stage near the inlet of the reactor 8 and that near the outlet of the reactor 8, and thus the degree of catalyst deterioration differs from one catalyst stage to another in the reactor 8. If detailed data on how the catalyst deterioration is distributed throughout the reactor 8 are available, the catalyst performance can be economically controlled by exchanging only the deteriorated catalyst, etc. However, to obtain such detailed data according to the conventional sampling procedure, catalyst test pieces 12 for sampling must be provided at so many positions throughout the reactor 8, complicating the structure of reactor 8 and consuming much labor and time in taking out the so many distributed catalyst test pieces 12 from the catalyst packages for the sampling. When the catalyst test pieces are taken out according to the conventional sampling procedure, catalyst must be filled into the sampled spaces to prevent a gas flow disturbance and maintain the performance. Thus, a larger amount of catalyst must be made ready for operation. Such a measure is actually quite impossible.